**Core Concept**
Insulinoma is a rare tumor of the pancreas that produces excess insulin, leading to hypoglycemia. Accurate diagnosis is crucial for managing this condition. The gold standard test for insulinoma involves measuring the body's ability to produce glucose in response to hypoglycemia.

**Why the Correct Answer is Right**
The gold standard test for insulinoma is the **Fasting Hypoglycemia Test with Glucose Tolerance Test (GTT)**. During this test, the patient is kept fasting for 12-18 hours, and blood glucose levels are measured. If hypoglycemia occurs, a glucose load is administered to assess the body's response. Normally, glucose levels should rise in response to the glucose load. In patients with insulinoma, glucose levels may not rise or may even decrease further, confirming the diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Insulin levels** may be elevated in insulinoma, but measuring insulin levels alone is not sufficient for diagnosing the condition. It can be elevated in other conditions as well, making it a non-specific test.
* **Option B:** **CT scan** or **MRI** may be used to locate the tumor, but they are not the gold standard tests for diagnosing insulinoma. They can help in localizing the tumor but do not confirm the diagnosis.
* **Option C:** **Oral glucose tolerance test (OGTT)** is used to diagnose diabetes and impaired glucose tolerance, but it is not the gold standard test for diagnosing insulinoma.

**Clinical Pearl / High-Yield Fact**
Insulinoma is a rare tumor, but it can be diagnosed with high accuracy using the Fasting Hypoglycemia Test with Glucose Tolerance Test. In patients with suspected insulinoma, it is essential to perform this test to confirm the diagnosis.

**Correct Answer:** C. Oral glucose tolerance test